Comparison of the binding affinity of CGP-12177A at recombinant rat α1D-adrenoceptors expressed in BHK-21 cell membranes and α1-adrenoceptors present in rat cerebral cortex membranes

Recent in vitro studies, performed in rat aorta, mesenteric and intrapulmonary arteries, and human pulmonary artery, demonstrated that the β-adrenoceptor ligand CGP-12177A (4-[3-[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one) is also provided with antagonist or partial agonist properties at α1-adrenoceptors. These observations were supported by estimates of CGP-12177A binding affinity at α1-adrenoceptors, which have been always performed in rat cerebral cortex membranes, as a surrogate of vascular tissue. Since α1D-adrenoceptors are predominant in both rat aorta and mesenteric artery, in the present study, we measured, for the first time, the binding affinity of CGP-12177A at recombinant rat α1D-adrenoceptors expressed in BHK-21 cell membranes. CGP-12177A binding affinity was also determined in rat cerebral cortex membranes, where various α1-adrenoceptor subtypes are present. By means of [3H]prazosin binding competition experiments, we found that CGP-12177A bound to α1D-adrenoceptor-expressing BHK-21 cell membranes, with a binding affinity (pKi = 5.39 ± 0.27) almost identical to that measured in cerebral membranes (pKi = 5.44 ± 0.07), indicating that it is a non-subtype selective α1-adrenoceptor ligand. Moreover, CGP-12177A binding affinity was very close to its functional affinity evaluated in rat aorta in terms of antagonist potency against phenylephrine-induced contraction (pKB = 5.65 ± 0.07). In conclusion, our results demonstrate that, in order to evaluate CGP-12177A binding affinity at aorta and mesenteric artery α1-adrenoceptors, estimates in rat cerebral membranes are as reliable as those in recombinant rat α1D-adrenoceptors, since both values are very close to CGP-12177A functional affinities in isolated vessels.